Post-menopausal changes in bone mineral content (BMC) and the effect of suppletory estrogen or 1,25(OH)D3 was studied. BMC was evaluated by means of quantitative computed tomography. Post-menopausal BMC (age 45-49) was 158.4 +/- 42.0 mg/cm3 which was significantly lower than in other subjects in the same aged group (192.3 +/- 27.1 mg/cm3). At two years, ten years and more than ten years post-menopause it decreased significantly by 17%, 30%, 55% respectively, compared with that of pre-menopausal BMC. The administration of conjugated estrogen (0.625 mg) or 1,25(OH)D3 was effective in preventing bone loss, although a transient decrease in BMC was observed in some cases to which 1,25(OH)D3 was administered. In cases of pre-menopausal bilateral oophorectomy, BMC was found to decrease significantly to the post-menopausal level about two years after operation. However, long-term estrogen replacement therapy tends to inhibit the decrease.